CAIE A-Level Biology AS 2.3 Proteins Topic Practice

Starch molecules are the main storage molecules in many types of cereal grain, such as the grain of the barley plant.

When the seed inside a barley grain germinates, genes coding for digestive enzymes are switched on. The enzymes that are synthesised catalyse the hydrolysis of storage molecules such as proteins and starch.

The hydrolysis of proteins in the barley seed produces amino acids that can be used in the synthesis of the proteins required for formation of the seedling (young plant).

Fig. 2.1 is an incomplete diagram of the molecular structure of the smallest amino acid, glycine. Each molecule of glycine has two carbon atoms.

Complete Fig. 2.1 to show the molecular structure of glycine.

Smooth muscle is a tissue composed of smooth muscle cells. The cells contain cytoplasm packed with proteins that are involved in contraction and relaxation.

Caldesmon is a large protein with a number of binding sites to attach to other proteins.

Caldesmon exists in two different forms, H-caldesmon and L-caldesmon.
H-caldesmon helps to regulate contraction and relaxation in smooth muscle cells.
L -caldesmon is found in some non-muscle cells, where it also acts as a regulatory protein.
- Caldesmon is coded for by a gene known as CALD1.
- CALD1 has 17 exons.
- The primary structure of H-caldesmon has a repeating sequence in the middle of the amino acid chain that is not present in L-caldesmon.

Suggest how the two different forms of caldesmon can still have similar functions, even though they have a different primary structure.

Rheumatoid arthritis (RA) is a disease of the joints in the human body.

The symptoms of RA include inflammation of the joints which causes pain and difficulty in movement of the joint.

The inflammation is triggered by a chemical known as TNF- $\alpha$, produced by macrophages.
One approach to the treatment of RA is by the use of monoclonal antibody against TNF- $\alpha$.
Fig. 3.1 is a diagram of an antibody molecule.

Name the type of bonds that hold the polypeptide chains together in the antibody structure.